1. Field of the Invention
The present invention relates to a coordinate input apparatus wherein position indicated on an input plane can be read at high accuracy.
2. Description of the Prior Art
In the prior art, an example of the coordinate input apparatus wherein position indicatcd on an input plane can be read at high accuracy is constituted in that scanning signal of the same phase is transmitted simultaneously into at least two conductors among a plurality of conductors embedded in parallel on a tablet, signal produced by the scanning signal transmitted into the conductors is detected by a coordinate indicating tool, inversion of polarity of the detected signal is discriminated, signal level is detected before and after the polarity inversion, and position of pointing by the coordinate indicating tool is operated and detected based on position of discrimination of the polarity inversion and the signal level.
FIG. 17 shows a basic constitution diagram of an example in the prior art, which will be described more specifically referring to FIG. 17. In FIG. 17, numeral 21 designates a switching circuit, numeral 22 conductors embedded in parallel on an input plane, numeral 23 a pickup for detecting in form of electric signal a magnetic field generated by high-frequency current flowing through the conductors, numeral 24 an amplifier, numeral 25 a polarity discrimination circuit for discriminating inversion of polarity of the signal detected by the pickup and amplified, numerals 26-1, 26-2 sample/hold circuits, numeral 27 an A/D converter, numeral 28 an oscillator for supplying high-frequency current to the conductors 22, numeral 29 a driver, numeral 30 a detection circuit, numeral 31 an adder, and numeral 32 a control device.
In FIG. 17, the coordinate input apparatus in the prior art performs at first step the rough detection of position of the pickup 23 regarding that it exists adjacent to which conductor 22. At second step, the coordinate input apparatus performs the detection at high accuracy regarding that it exists at which position between the conductor 22 and an adjacent conductor for example. The detection will be hereinafter described in detail.
The rough detection at first step will be described. Required data is transmitted from the control device 32 into the switching circuit 21. Based on the informed data, the switching circuit 21 changes in sequence the high-frequency current generated by the oscillator 28 and the driver 29 so that two conductors selected from the conductors 22 in every other conductor such as A and C, B and D, C and E, D and F . . . are scanned in the same current. The magnetic field generated by the conductors 22 is detected in form of electric current by the pickup 23 and amplificd by the amplifier 24. The amplified signal is supplied to the polarity discrimination circuit 25, and discrimination is performed regarding whether or not the polarity is inverted corresponding to any scanning position. If the polarity inversion is discriminated, this is informed to the control device 32. Thus it follows that the rough position of the pickup 23 is detected.
The detection of high accuracy at second step will be described. The control device 32 at receiving information of the polarity inversion supplies hold pulse to the sample/hold circuit 26-1. Signal V.sub.2 amplified and detected through the amplifier 24 and the detection circuit 30 and then inputted is held and stored by the sample/hold circuit 26-1. The control device 32 produces information to the switching circuit 21, and returns the state by one step. The hold pulse is supplied to the sample/hold circuit 26-2. Signal V.sub.1 amplified and detected through the amplifier 24 and the detection circuit 30 and then inputted is held and stored by the sample/hold circuit 26-2. The signals V.sub.1 and V.sub.2 stored in this manner are supplied to the adder 31, and the sum signal (V.sub.1 +V.sub.2) is calculated. The sum signal (V.sub.1 +V.sub.2) is supplied to reference voltage input terminal V.sub.REF of the A/D converter 27, and either of the signals V.sub.1, V.sub.2, for example, the signal V.sub.1 is supplied to input terminal V.sub.IN. Thereby following value is calculated. ##EQU1##
Value of denominator and value of numerator in formula (1) are inputted respectively to the reference voltage input terminal V.sub.REF and the input terminal V.sub.IN of the A/D converter 27, thereby the value of formula (1) can be calculated without using divider of high cost. Since the signal V.sub.1 is normalized by value of the sum signal (V.sub.1 +V.sub.2) of denominator, the input coordinate value X.sub.i can be calculated always stably even if the detection voltage varies. The calculated input coordinate value X.sub.i is informed to the control device 32. The input coordinate value Y.sub.i is calculated in similar manner, and informed to the control device 32. Based on the input coordinate values (X.sub.i, Y.sub.i), the position coordinates (X, Y) where the pickup on the input plane exists can be calculated and outputted.
In the calculation method of the prior art, the two conductors 22 are selected and the polarity inversion at intermediate point between the two conductors is assumed thereby the calculation is performed. However, the inversion position may be shifted from the intermediate point due to the magnetic field generated from the common conductor supplying current to each conductor 22, thereby the calculation error due to the shift becomes a problem. Consequently, in another constitution, the compensation loop is installed to surround the conductors 22 and elimination of the influence of the common conductor is intended. Also in this constitution, since the influence cannot be completely eliminated, interpolation considering the shift of the polarity inversion position is necessary. Particularly, in constitution that each one loop is selected in sequence and scanned, the shift becomes a problem in accuracy. This constitution will be described specifically.
FIG. 9 shows a detection voltage distribution when there is no shift of the polarity inversion position. Here is shown an example of distribution of region of 10 mm .ltoreq.X .ltoreq.30 mm, and each loop inverting at X =10, 15, 20, 25, 30 mm is designated by L.sub.10, L.sub.15, L.sub.20, L.sub.25, L.sub.30. Distribution of the magnetic field intensity H.sub.z corresponding to each loop L.sub.10 .about.L.sub.30 becomes positive at the left side of the zero point and becomes negative at the right side thereof, and the voltage detects absolute value of the magnetic field intensity H.sub.z. Regions of 10 mm.ltoreq.X.ltoreq.20 mm, 15 mm.ltoreq.X.ltoreq.25 mm, 20 mm.ltoreq.X.ltoreq.30 mm are made rough regions and called segment 2 (S.sub.2), segment 3 (S.sub.3) and segment 4 (S.sub.4), respectively.
Assuming that the pickup exists at T position (X=21 mm approximately) in FIG. 9, until when S.sub.4, X=20 mm, H.sub.z &lt;0 (T position being to the right of the zero point in L.sub.20 curve), and when S.sub.5, X=25 mm, H.sub.z &gt;0. Consequently, the detection voltage V.sub.2 due to the loop of L.sub.25 is obtained. The forward segment by two, that is, loop L.sub.15 of S.sub.5-2 =S.sub.3, X=25-10=15 mm, is selected and the detection voltage V.sub.1 due to this is obtained. In this example, segment S.sub.3 corresponding to region of 15 mm.ltoreq.X.ltoreq.25 mm is selected as segment of interpolation.
Next, an example at shifting of the polarity inversion position is shown in FIG. 10. In this example, distribution curve of the magnetic field intensity H.sub.z as above described is shifted to the positive direction of X, and like parts to the above loops and segments are designated by the same reference numerals. Also in this example, the pickup exists at T position adjacent to X=21 mm.
In this case, when S.sub.4, X=20 mm, H.sub.z &gt;0 already. Consequently, the interpolation region becomes S.sub.4-2 =S.sub.2, and the interpolation is performed at region of 10 mm.ltoreq.X.ltoreq.20 mm.
In other words, in spite of shifting out of the original segment region, the interpolation calculation is performed at the shifted region resulting in the error operation.
In this example of the prior art, since at least two conductors are selected and the scanning signal of the same phase must be transmitted into these conductors, the independent driver of constant current amplitude is required per each one conductor, and further two transistors, two registers, one decision register and one operation amplifier are required per one conductor, thereby the cost of circuits becomes high.
Furthermore, since a number of parts are used, control of variation in manufacturing of parts is difficult and adjustment after assembling the parts is troublesome.
Since a cheap decoder IC (output being always one line) cannot be used in the scanning, the apparatus must be constituted by a shift register. If the shift register is used, time is required before driving an aimed conductor resulting in another problem.